1. Field of the Invention
The present invention specifically relates to a transmission apparatus and polar modulation method using a polar modulation scheme.
2. Description of the Related Art
Upon designing a transmission modulation apparatus in digital radio communication, high efficiency is generally required. However, for the high frequency power amplifier (“PA”) having the largest power consumption in a transmission modulation apparatus, linear characteristics of little distortion are further required in addition to efficiency. For these requirements, a technique has been proposed that enables both efficiency and linearity in a transmission modulation apparatus by using a polar modulation scheme. With the polar modulation scheme, even when a high frequency power amplifier is utilized in a saturation region of high efficiency, an linear amplification is possible.
An outline of a polar modulation scheme will be explained. FIG. 1 is a block diagram showing a sample configuration of a transmission modulation apparatus employing a polar modulation scheme. Polar coordinate conversion section 11 divides a modulation signal into the amplitude signal representing the amplitude component (for example, √I2+Q2) and the high frequency phase modulation signal representing the phase component (for example, the angle formed by the modulation symbol and the I-axis). The amplitude signal is provided to the power supply voltage of transmission power amplification section 13 formed with a high frequency power amplifier. After the power of the high frequency modulation signal is adjusted in power adjustment section 12 (formed with, for example, a variable gain amplifier and variable gain attenuator) based on a power adjustment signal for controlling the power of the high frequency phase signal, the high frequency modulation signal is outputted to transmission power amplification section 13. Transmission power amplification section 13 amplifies the power of the high frequency phase modulation signal using the amplitude signal as the power supply voltage. Thus, while amplifying the high frequency phase modulation signal in a state where transmission power amplification section 13 is operated in a saturation region, by attaching the amplitude component to the high frequency phase modulation signal and generating a transmission signal based on fluctuation of the power supply voltage, it is possible to realize both high linearity and high efficiency at the same time.
Generally, in the case of taking into account the operation environment of a transmission modulation apparatus, when the ambient temperature changes, the characteristics of the high frequency power amplifier forming transmission power amplification section 13 fluctuate. For example, for a high frequency power amplifier formed with an HBT (Heterojunction Bipolar Transistor), a change in temperature makes the relationship fluctuate between the power supply voltage provided to the power supply voltage of the high frequency power amplifier and the output power. That is, even when the same power supply voltage is provided, output power fluctuates based on temperature. Due to this, linear characteristics of the high frequency power amplifier degrade, and, consequently, a problem arises that, for example, jamming signals to adjacent frequency bands are produced. Therefore, in response to such temperature change, adaptive temperature compensation needs to be performed.
Patent Document 1 (Japanese Patent Application Laid-Open No. 2007-130782) discloses a polar modulation transmission apparatus that implements temperature compensation. The polar modulation transmission apparatus of Patent Document 1 acquires temperature information by a temperature sensor and performs temperature compensation for the amplitude signal according to the temperature information. By this means, when the temperature changes, it is possible to compensate characteristic degradation due to fluctuation of a relationship between the power supply voltage and output power of a high frequency power amplifier.
By the way, radio communication schemes such as UMTS (Universal Mobile Telecommunication System) are demanded to control the output power of a transmission signal over a quite wide range.
However, studies are not sufficiently underway for a polar modulation transmission apparatus that can control the output power of a transmission signal over a wide range and suppress degradation of signal quality due to temperature change.